1. Field of the Invention
This invention relates to a flaw detection method and a flaw detection apparatus. More particularly, the invention relates to those useful when applied in making a nondestructive inspection of a member, which is a magnetic material and is a subject for flaw detection, by generating eddy currents in the member by an alternating magnetic field generated by a coil, and examining a change in a magnetic flux density due to the eddy currents disturbed by the presence of a flaw.
2. Description of the Related Art
The eddy-current flaw detection method is known as a flaw detection method which detects a flaw on the surface or in the interior of a metallic material used in a component of electric power equipment (for example, a gas turbine blade of thermal power generating system). This eddy-current flaw detection method detects a flaw by generating eddy currents in a member as a subject for flaw detection (hereinafter referred to as a member to be flaw-detected) by an alternating magnetic field generated by a coil, and detecting a change in a magnetic flux density due to the eddy currents disturbed by the presence of the flaw.
The above-mentioned flaw detection method will be described in further detail with reference to FIGS. 20A to 20D showing the principle of this method. As shown in FIG. 20A, a magnetic flux 03 is generated by flowing an electric current 02 through a coil 01. Then, as shown in FIG. 20B, the coil 01 is approached by a member 04 to be flaw-detected, which is a magnetic material, to generate eddy currents 05 in this member 04 to be flaw-detected. As a result, a magnetic flux 06 due to the eddy currents 05 is generated, as shown in FIG. 20C. If, at this time, a flaw 07 exists in the member 04 to be flaw-detected, as shown in FIG. 20D, the eddy currents 05 are disturbed. Consequently, the magnetic flux 06 by the eddy currents 05 changes. Based on such a change in the magnetic flux 06, the flaw 07 of the member 04 to be flaw-detected is detected.
In recent years, however, a further improvement in flaw detection accuracy has been desired. The accuracy of the eddy-current flaw detection method according to the earlier technology has proved insufficient, particularly when identifying the location of the flaw.
A proposal has been made for a flaw detection method which detects a flaw in a member to be flaw-detected, which is a magnetic material, by detecting a change in a magnetic flux density as a change in the rotation angle of the plane of polarization with the use of Faraday effect (a phenomenon in which when linearly polarized light passes through a magnetized crystal, the plane of polarization rotates). An example is the flaw detection method disclosed in Japanese Patent Application Laid-Open No. 1990-189457.
According to this flaw detection method, as shown in FIGS. 21 and 22, a magnetic field is imparted by a magnetization means 013 to a portion, including a test surface 012, of a test sample 011 comprising a magnetic material. In this state, the one of the test sample 011 and a sensor-unit 015, which is disposed in the vicinity of the test sample 011 and incorporates a Faraday element 016, is moved. Simultaneously, light L is enters the Faraday element 016, and the optical intensity of the light L, which exits from the Faraday element 016, is measured to detect fluctuations in a magnetic flux leakage A from the test sample 011, thereby detecting a defect region 014 of the test sample 011. The sensor unit 015 integrally accommodates a polarizer 017, an analyzer 018, and a mirror 019, as well as the Faraday element 016, inside a housing.
Thus, the light L emitted from an optical transmitter 020 arrives at the polarizer 017 via an input optical fiber 021, reaches the outside of the sensor unit 015 via the Faraday element 016, the analyzer 018 and the mirror 019, and becomes incident on an optical receiver 023 via an exit optical fiber 022. The light L incident on the optical receiver 023 is converted into an electrical signal θ, and processed in a predetermined manner by a computing unit 024, whereafter the result of processing is indicated on a display unit 025 such as an X-Y recorder.
According to the above-described flaw detection method, however, the light L, which has been incident on the Faraday element 016 via the polarizer 017, crosses the Faraday element 016 and enters the analyzer 018. Thus, the light L is influenced by the magnetic field in the entire region, beginning at the site of its entry into the Faraday element 016 and ending at the site of its departure from the Faraday element 016. Accordingly, the region for measurement of the magnetic flux density widens, resulting in poor resolution.
As documents on publicly known technologies relevant to the present invention, the aforementioned laid-open patent application and Japanese Patent Application Laid-Open No. 1994-294773 can be named.
The present invention has been accomplished in light of the above-described problems with the earlier technologies. It is an object of the present invention to provide a flaw detection method and a flaw detection apparatus which are applied when detecting a flaw in a magnetic material, as a member to be flaw-detected, by detecting a change in a magnetic flux density as a change in the rotation angle of the plane of polarization with the use of Faraday effect, and which can impart high resolution to a measurement region for magnetic flux density to achieve flaw detection with high accuracy.